Fluoropolymers have been long known and have been used in a variety of applications because of several desirable properties such as heat resistance, chemical resistance, weatherability, UV-stability, low friction and anti-stick properties, etc. The various fluoropolymers are described, for example in “Modern Fluoropolymers”, edited by John Scheirs, Wiley Science 1997 or in “Fluoropolymers” edited by Sina Ebnesajjad, Plastics Design Library, Norwich, N.Y., 2000.
Commonly known or commercially employed fluoropolymers include polytetrafluoroethylene (PTFE), copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) (such copolymers are also referred to as FEP polymers), copolymers of tetrafluoroethylene and perfluoroalkoxy copolymers (such copolymers are also referred to as PFA), copolymers of ethylene and tetrafluoroethylene (such copolymers are also referred to as ETFE polymers), copolymers of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride (VDF) (such copolymers also referred to as THV) and polyvinylidene fluoride polymers (PVDF) and others.
Fluoropolymers have been used to improve the thermal and chemical resistance of a substrate or to provide anti stick or low friction properties to a substrate, for example by coating or impregnating the substrate with the fluoropolymers. The fluoropolymers may be applied to the substrate by liquid coating techniques if they are provided as a liquid formulation, such, as for example, as dispersions.
Fluoropolymer dispersions can be conveniently produced by aqueous emulsion polymerization employing fluorinated monomers, one or more radical reaction initiators and a suitable emulsifier. Perfluorinated alkanoic acids of the type CF3—(CF2)n—COO− with n being an integer of 6 to 8, and in particular perfluorooctanoic acid (n=6) and its salts, have been the emulsifier of choice in the aqueous emulsion polymerization of fluoromonomers for the last decades. Using perfluorooctanoic acid increases the solid content (polymer content) of the dispersions and leads to the generation of polymers of desired molecular weight and particle size in acceptable reaction rates and to stable dispersions. However, perfluorinated emulsifiers are expensive materials. Some perfluorinated acids have also been found to be poorly bio-degradable. Therefore, methods have been developed to retain and recycle the fluorinated emulsifiers from fluoropolymer dispersions leading to dispersion of low content of fluorinated emulsifiers.
Another attempt to reduce the amount of fluorinated emulsifiers in fluoropolymer dispersions involves preparing the fluoropolymers without using fluorinated emulsifiers. Therefore emulsion polymerization methods not requiring fluorinated emulsifiers at all have become of increased interest.
An aqueous emulsion polymerization without using any surfactants has been disclosed, for example, in U.S. Pat. No. 5,453,477, WO 96/24622 and WO 97/17381. For example, WO 97/17381 reports an aqueous emulsion polymerization in the absence of a surfactant wherein a radical initiator system of a reducing agent and oxidizing agent is used to initiate the polymerization and whereby the initiator system is added in one or more further charges during the polymerization.
Methods using non-fluorinated emulsifiers have also been described, for example in U.S. Patent Publ. No. 2006/0281845, in EP 1 462 461 or in WO 2008/033271. In WO 2008/033271 small particle dispersions of TFE, HFP and VDF copolymers with a major part of the polymer made up by TFE were obtained using a carbosilane as emulsifier. However, carbosilanes are not readily available and their use at present is not economical.